Environmentally driven regulatory pressures concerning motor gasoline (“mogas”) are expected to increase demand for mogas having no more than about 50 wppm sulfur, and preferably less than about 10 wppm. In general, this will require deep desulfurization of olefinic cracked naphthas, such as cat naphthas. That is, naphthas resulting from cracking operations that typically contain substantial amounts of both sulfur and olefins. Deep desulfurization of cat naphtha requires improved technology to reduce sulfur levels without the severe loss of octane that accompanies the undesirable saturation of olefins.
Hydrodesulfurization is a hydrotreating process for the removal of feed sulfur by conversion to hydrogen sulfide. Conversion is typically achieved by reaction of the feed with hydrogen over non-noble metal sulfided supported and unsupported catalysts, especially those of Co/Mo and Ni/Mo. Severe temperatures and pressures may be required to meet product quality specifications or to supply a desulfurized stream to a subsequent sulfur sensitive process.
Olefinic cracked naphthas and coker naphthas typically contain more than about 20 wt. % olefins. At least a portion of the olefins are hydrogenated during conventional hydrodesulfurization. Since olefins are relatively high octane number species components, it may be desirable to retain the olefins rather than to hydrogenate them to saturated compounds. Conventional fresh hydrodesulfurization catalysts have both hydrogenation and desulfurization activity. Hydrodesulfurization of cracked naphthas using conventional naphtha desulfurization catalysts following conventional startup procedures and under conventional conditions required for sulfur removal, produces a significant loss of olefins through hydrogenation. This results in a lower grade fuel product that needs additional refining, such as isomerization, blending, etc., to produce higher octane fuels. This, of course, adds significantly to production costs.
Selective hydrodesulfurization involves removing sulfur while minimizing hydrogenation of olefins and octane reduction by various techniques, such as selective catalysts, process conditions, or both. For example, ExxonMobil's SCANfining process selectively desulfurizes catalytically cracked naphthas with little loss in octane. U.S. Pat. Nos. 5,985,136; 6,013,598; and 6,126,814, all of which are incorporated by reference herein, disclose various aspects of SCANfinig. Although selective hydrodesulfurization processes have been developed to avoid olefin saturation and loss of octane number, such processes can liberate H2S that reacts with retained olefins to form mercaptan sulfur by reversion.
Stricter mogas sulfur regulations will also make it necessary to desulfurize certain virgin naphtha streams that have in the past been directly blended into the mogas pool without being hydrodesulfurized because of their relatively low sulfur content. Mild hydrotreating technology to reduce virgin naphtha sulfur to very low levels with no significant loss of octane number is known, but the construction and operation of an additional hydrotreater dedicated to this service would be undesirably costly.
Consequently, there is a need for technology that will reduce the cost of hydrotreating both olefinic cracked naphthas and naphthas that are substantially free of olefins.